This invention relates to electrical cables. More particularly it relates to an insulated electrical conductor for use in high temperature, high pressure gassy oil wells adapted to prevent depressurization-caused rupture failures from occurring in service and during removal from the well bore, and to cables constructed thereof.
Prior art cable structures employed for oil well service include those having conductors of stranded copper, separately insulated with a material of high dielectric strength. To protect the insulated conductors from attack by well fluids, they are sheathed in an extruded elastomeric jacket adapted to resist penetration by the well fluids. Typical of these prior art cables is the structure disclosed in U.S. Pat. No. 3,485,939, having three conductors of stranded copper separately insulated and helically wound and sheathed in an extruded jacket of nitrile rubber or a similar elastomer. The rubber jacket is surrounded by an outer armor formed of a continuous wrapped band of a metallic material. The outer armor does not provide an hermetic seal, and exclusion of well fluids from the internal structure is intended to be accomplished by means of the water-resistant and hydrocarbon-impervious jacketing material. Invasion of these cable structures by low molecular weight well fluids occurs particularly when the cables are employed in highly gassy wells, either through gas permeation or by way of pinholes and other defects in the jacket. Although the jacketing and insulation layers of these prior art cables have been designed to resist chemical attack and deterioration when permeated by well fluids, the presence of low molecular weight hydrocarbons and gases under high pressure within the interstices of the cable structure frequently causes mechanical destruction such as blow outs and rupture failures. These ruptures are particularly likely to occur when the external pressure on the cable is decreased, as for example when the cable is removed from the well bore, inasmuch as these cables are not designed to withstand an unbalanced high internal pressure condition. The high internal pressures induce ballooning of the insulation and the jacketing material which then burst and rupture the metal armor, rendering the cable useless.
An improved prior art cable design, disclosed in U.S. Pat. No. 3,710,009, includes an extruded outer armor formed of a water-impervious, high temperature, heat-resistant polyolefin. The extruded armor provides a further mechanical barrier to invasion by well fluids. In practice it has been found that the interstices of these cable structures can also be penetrated by the low molecular weight hydrocarbons in highly gassy wells either by way of gas permeation or through defects and these cable structures similarly suffer from blow-out and rupture of the insulation, jacketing and armor, particularly during rapid depressurizing.
In a third prior art cable construction, disclosed in U.S. Pat. No. 3,835,929, the jacketed well cable is encased in a continuous metallic tube, sealed at the lower end and extending to the surface. Such constructions are difficult to employ in the field, and require cables having tensile strength sufficient to withstand installation in an unsupported manner through the entire length of the tubing. Additionally, intrusion of low molecular weight well fluids such as methane under high pressure through a defective seal means at the lower end would result in a fluid-filled conduit which turn would make subsequent removal of the cable both difficult and hazardous.
Other methods for protecting well cables from invasion by highly pressurized low molecular weight well fluids have also generally relied on a combination of materials to enhance the cable's mechanical resistance to penetration. It has not heretofore been possible to completely prevent gas permeation of these cables when under very high pressure, and a cable structure for use in gassy wells capable of withstanding permeation by low molecular weight well fluids without consequent ruptures and blow out failures would clearly be a welcome advance in the art.